Marker-assisted Selection Of QMrdd8 To Improve Maize Resistance To Rough Dwarf Disease

Maize rough dwarf disease?MRDD?is a viral disease worldwide.In recent years,MRDD has been highly prevalent in the Yellow-Huai-Hai River maize-growing areas in China.Genetic resistance is an economic and effective approach to preventing the disease.MRDD resistance can be improved by marker-assisted selection in combination with conventional breeding strategies.In our previous study,qMrdd8,a major QTL that confers resistance to MRDD was finely mapped into an interval defined by markers IDRQ2 and IDRQ20,and a functional marker IDP25K was further developed within the candidate gene.In this study,qMrdd8 from a donor parent X178 was introgressed into the major maize inbred lines using multi-generation backcrossing and marker-assisted selection,and the genetic effect of qMrdd8 and the marker selection effect were evaluated in a synthesized population ZDP1,which would provide a basis for germplasm improvement against MRDD.The main results were as follows:1.The resistant inbred line X178 was used as a donor parent,and the susceptible lines Mo17,Ji846,Huangzaosi,Chang7-2,Ye478,and Zheng58 were used as recurrent parents.qMrdd8 was introgressed into the susceptible inbred lines through crossing,multi-generation backcrossing,and self-pollination.In this process,foreground and background selection of plants in backcrossing populations were conducted using molecular markers,and homozygous resistant lines Huangzaosi?R?,Chang7-2?R?,Ye478?R?,and Zheng58?R?were obtained.2.A total of 107 genome-wide SSR markers were used for background selection of BC₃F₁ plants of Mo17 and Ji846 and BC₄F₁ plants of Huangzaosi,Chang7-2,Ye478,and Zheng58.For BC₃F₁ plants of Mo17 and Ji846,the highest recovery rates of the recurrent parent genome were 89.63%and 95.47%,respectively.For BC₃F₁ plants of Huangzaosi,Chang7-2,Ye478 and Zheng58,the highest recovery rates of the recurrent parent genome were 98.02%,97.52%,96.74%and 95.70%,respectively.3.Inbred lines X178,Mo17,Ji846,Huangzaosi,Chang7-2,Ye478,Zheng58,and plants from different generations of backcrossing were evaluated for MRDD resistance under natural infection at Xuzhou in Jiangsu and Xinxiang in Henan.As a result,X178 was highly resistant to MRDD and the disease severity index?DSI?was 11.08%,and DSIs of heterozygous plants with qMrdd8 were reduced by 9.32%to 22.81%and 9.87%to 22.16%,respectively,compared to the respective susceptible plants and recurrent parents.The marker IDP25K could screen precisely the plants habouring qMrdd8 alleles in foreground selection.4.The improved inbred lines Huangzaosi?R?,Chang7-2?R?and their hybrids were evaluated for MRDD resistance under natural infection and artificial inoculation at Yuanyang in Henan and Nanjing in Jiangsu,respectively.Under natural infection,DSIs of Huangzaosi?R?and the hybrids Mo17/Huangzaosi?R?,Ji846/Huangzaosi?R?,and Huangzaosi?R?/Ji846?R?were reduced by 26.14%,12.96%,17.38%,and 27.19%,respectively,compared to those of the controls.DSIs of Chang7-2?R?and the hybrid Zheng58?R?/Chang7-2?R?were reduced by 18.75%and 17.05%,respectively.Under artificial inoculation,DSIs of Huangzaosi?R?and Chang7-2?R?were reduced by 19.72%and 8.75%,respectively.DSIs of the hybrids Mo17/Huangzaosi?R?and Ji846/Huangzaosi?R?were reduced by12.96%and 17.38%,respectively.DSIs of Huangzaosi?R?/Ji846?R?and Zheng58?R?/Chang7-2?R?were reduced by 12.57%and 18.81%,respectively.No significant difference in agronomic traits,including plant height,ear height,total tassel length,tassel branch number,upper and lower internode number of ear,was observed between isogenic inbred line pairs and hybrids?P>0.05?.5.The frequencies of qMrdd8 alleles in three cycles of improved ZDP1?C₂,C₃,and C₄?were analyzed using the marker IDP25K.Among C₂,C₃,and C₄,the frequencies of qMrdd8 alleles were4.43%,6.47%,and 7.89%,respectively,and their DSIs were 49.72%,44.40%,and 41.98%,respectively.The three original improved populations were classified as resistant populations?RC₂,RC₃,and RC₄?and susceptible populations?SC₂,SC₃,and SC₄?by marker genotyping.DSIs of RC₂,RC₃,and RC₄were significantly lower than those of SC₂,SC₃,and SC₄ and C₂,C₃,and C₄,with their DSIs decreased by 6.82%to 15.45%.Comparative analyses of agronomic traits in the same generation of population indicated that no significant difference was found in six plant architecture traits and seven ear-related traits?P>0.05?among the three original populations and their resistant or susceptible populations.